Disease response of carrot and carrot somaclones to Alternaria dauci

Abstract

Leaf spot of carrot, caused by the fungus Alternaria dauci, is a common disease of carrot throughout the world. To assess the disease response in different genotypes an in vitro method of assay has been developed. The terminal portion of excised mature leaves from carrot are incubated under lights at 25–26°C for 6 days with spores of A. dauci. After 6 days, chlorophyll and total soluble polyphenol contents of the leaves decline to a low level, whereas in the uninoculated control leaves levels of these compounds are largely unchanged. Electron microscope studies of the effect of infection on the ultrastructure of leaves from both cv. Fancy and wild plants show disintegration of the cell wall, plasmalemma and chloroplast membranes in the region close to the invading fungus. The response to A. dauci shows characteristics compatible with the presence of a phytotoxin. Seedling hypocotyls of the carrot cv. Fancy are used as a source of explants to initiate callus tissue and regenerate embryos. Regenerant somaclone plants show variation in morphology (crown number, height, leaf number, leaf shape and flowering time) and susceptibility to the pathogen as measured by the loss in chlorophyll from infected detached leaves after a 6-day period. Potentially susceptible and resistant regenerant plants are identified by the low and high chlorophyll levels, respectively, in excised leaves 6 days after infection. Selected high- and low-chlorophyll regenerant plants and seedling-derived cv. Fancy are self-pollinated, then the seeds are used to produce a progeny generation, which is assessed for morphological changes and disease resistance as before. The response of leaves of progeny plants to infection shows that the plants derived from high-chlorophyll regenerants still show a higher chlorophyll level after infection compared with leaves derived from low-chlorophyll and control plants. High chlorophyll levels in the presence of infection in the progeny of high-chlorophyll regenerants suggest that the capacity to resist infection in the selected somaclones is inherited.